1. Technical Field
The present invention relates to an image processing method, an image processor, and an image display system.
2. Related Art
As a technique for improving resolution and brightness of a projector, a stack display technique in which images projected from a plurality of projectors are stacked to display one image is known. Similarly to an image displayed by a single projector, a stack image has luminance unevenness or color unevenness. In general, correction of luminance unevenness or color unevenness is performed on the entire screen based on the darkest portion within the screen when white display is performed (that is, when an image of white which is the brightest image from among images to be displayed by a projector is displayed). As a result of correction, brightness of white is degraded.
FIGS. 28A and 28B are explanatory views of a general unevenness correction principle. FIG. 28A shows an example of a change in luminance in a horizontal direction of a screen as the state of luminance unevenness when white display is performed for simplification of description. FIG. 28B schematically shows the change in luminance of FIG. 28A in a stepwise manner. In FIGS. 28A and 28B, the horizontal axis represents the pixel position of the screen and the vertical axis represents luminance (in a broad sense, intensity).
The change in luminance shown in FIG. 28A when white display is performed is viewed as image unevenness. In this case, in general, correction is performed such that the luminance of each position has the minimum value Ymin when the minimum value Ymin of luminance shown in FIGS. 28A and 28B is set as a target value for unevenness correction. This is because, when white display is performed, the whole portion may not be increased in luminance, such that, if the luminance of a portion other than the darkest portion is set as a target value for unevenness correction, the luminance of the darkest portion may not conform to the target value. For this reason, unevenness correction for improving display quality is performed in a direction in which brightness of white is degraded.
A projector has an inherent color characteristic, and if no correction is performed, the display characteristic may be considerably shifted from the standard colors, such as sRGB (standard RGB). If correction (color correction) is performed to match the colors to the target color, this causes the degradation of brightness of the projector.
FIG. 29 is an explanatory view of a general color correction principle. FIG. 29 schematically shows the state where a target output luminance characteristic to be displayed and an actual characteristic (actual device characteristic) of a projector are associated with each other using LUT (Look-Up Table) data in which unevenness correction values are tabulated.
For example, in order to realize transmittance T0 when a voltage V0 is applied, it is necessary to apply a voltage V1 according to the actual characteristic inherent in the projector. Accordingly, a search is carried out for the actual characteristics (input/output characteristics) of the projector measured in advance to create LUT data shown in FIG. 29. In the color correction, the voltage for realizing transmittance T0 is corrected to a voltage specific to the projector using LUT data. When a single liquid crystal panel as an optical modulator is provided, LUT data can be set directly by a single search. Meanwhile, in a projector using color composition of RGB, each of RGB has XYZ components, LUT data of R, G, and B may not be simply obtained from the search results of X, Y, and Z, and it is necessary to perform a search taking color mixture into consideration.
Various methods of improving luminance unevenness and color unevenness appropriate for a stack image have been suggested. For example, JP-A-2005-352171 describes a technique which calculates an intensity profile totaled for respective light source colors (RGB) and performs color unevenness correction such that evenness is achieved in a stack state where a stack image (superimposed image) is displayed, instead of performing color unevenness correction for respective projectors. According to the technique described in JP-A-2005-352171, it is not necessary that color unevenness is reduced for the respective projectors, and it should suffice that color unevenness of a stack image is reduced even if the individual images are uneven in color.
There is demand for power saving in an electronic apparatus, such as a projector. Accordingly, a projector which has a dimming function of adjusting brightness of a light source, thereby displaying an image while reducing power consumption is considered.
On the other hand, unevenness correction and dimming of the light source may cause degradation in brightness. These techniques are independent, and at worst, the degradation in overall brightness can be expressed by simply multiplying a reduction ratio of brightness according to the degree of unevenness correction, a reduction ratio of brightness according to the degree of color correction, and a reduction ratio of the light source. For example, when brightness becomes 60% by color correction, brightness becomes 70% by unevenness correction, and the light source is reduced to 60% so as to achieve low power consumption of 40%, this means that the amount of light loss of 75% (≅1−0.6×0.7×0.6) is caused.
Focusing on brightness after color correction and dimming on a stack image displayed by a first projector PJ1 and a second projector PJ2, even if the technique described in JP-A-2005-352171 is applied, an image reduced through just dimming is obtained. For this reason, even if the technique described in JP-A-2005-352171 is applied, there is a problem in that a stack image may not be displayed with optimum light use efficiency when dimming is performed.